1. Field of the Invention
The present invention relates to a Plastic optical fiber. More specifically, the present invention relates to a plastic optical fiber which can be used as optical fiber codes and optical fiber cables.
2. Description of the Related Art
Inorganic glass optical fibers have been known as optical fibers which are excellent in light transmission properties over a broad range of wavelengths. However, since said glass optical fibers are not good in processability or flexural strength, plastic optical fibers were developed and have been widely used as optional fibers.
These plastic optical fibers are basically comprised of a core polymer comprising a polymer which has excellent light transmission properties and a high refractive index, such as polymethyl methacrylate (hereinafter referred to as PMMA), polycarbonate (hereinafter referred to as PC), and a clad polymer comprising a transparent polymer which has a refractive index lower than that of the core polymer, such as fluorine-containing polymers.
Known examples of the plastic optical fibers of this type are optical fiber strands, bulk fibers made by covering optical fiber strands with a functional protective layer, optical fiber codes made by covering optical fiber strands with a jacket, bundle fibers made of an assembly of bulk fibers, and optical fiber cables made by applying tension members to the bulk fibers.
These plastic optical fibers, however, have many C--H bonds in the core polymer, and light absorption based on the stretching vibration of the C--H bonds, appears at the regions of the short wavelengths. Five to eight times harmonic absorptions also appear at the near infrared to visible ray region, namely, at a wavelength of not less than 400 nm. These serious light transmission losses in these regions have to be eliminated to use these conventional plastic optical fibers for light transmission. For example, the transmission loss of an optical fiber having a core of PMMA is about 100 dB/Km at a wavelength of 650 nm, and about 400 dB/Km at a wavelength of 780 nm. To avoid the transmission losses based on the C--H bonds in the core polymer, a core polymer comprising d.sub.8 --PMMA, of which all the H atoms in PMMA are replaced by deuterium atoms, was proposed. This optical fiber containing d.sub.8 --PMMA as a core polymer has a transmission loss of 50 dB/Km at a wavelength of 780 nm. Deutrated PMMA, however, has high water absorbing properties, and the d.sub.8 --PMMA core polymer absorbs water, and the transmission loss will increase with the lapse of time. An optical fiber showing such an increase in transmission loss cannot be used as an optical fiber, as an optical fiber is expected to have a high reliability over a long period.
At present light-emitting diodes (LEDs) that can emit rays in the near infrared region, and which have high power, and which can be used for high-speed data transmission, have been produced in large quantities at a low cost. Since conventional plastic optical fibers, however, cannot use these LEDs as a light source for optical communications, light transmission beyond a wave guide length longer than 100 m cannot be accomplished with one optical fiber. Thus, LAN systems (Local-Area Network Systems) using plastic optical fibers have not been so widespread.
Recently, plastic optical fibers that can transmit rays in the near infrared region have been developed. For example, an optical fiber comprising a core polymer of a polymer of .alpha.-fluoro acrylic acid fluoroalkyl ester, and a clad copolymer of vinylidene fluoride and tetrafluoroethylene was disclosed in EP 340557 A2 and EP 340555 A2. This optical fiber can transmit rays having a wavelength in the near infrared region, but its performance as an optical fiber is not satisfactory, since the difference in the refractive index between the core polymer and the clad polymer is not large enough to make an optical fiber having a large numerical aperture, and thus this optical fiber is not satisfactory as an optical fiber for transmitting data in a great amount. Further, because of its small numerical aperture, it is difficult for this optical fiber to inhibit the leakage of rays from its side surface when it is bent, and thus it is not satisfactory as an optical fiber for data communication.
Furthermore, the vinylidene fluoridetetrafluoroethylene copolymer is not a perfectly amorphous polymer, and by this reason, has light-absorbing properties or light-scattering properties. Thus, an optical fiber containing this clad copolymer is not satisfactory in light transmission properties.
As a transparent clad material having a low refractive index, a copolymer of perfluoro[2,2-dimethyl-1,3-dioxole] and at least one other ethylenically unsaturated monomer has been reported.
This clad material is proposed to be used in combination with a core material composed of an .alpha., .beta.-unsaturated carboxylic acid ester represented by the following general formula (I) as the main component: ##STR2## (wherein X is CH.sub.3, H, D, F, Cl, or CF.sub.3 ; Y is H or D; and Rf is a fluoroalkyl group having a linear or branched chain).
However, this core material is glassy and hard, and it is unsatisfactory in its strength for use as an optical fiber.
To use this core material as an optical fiber, it is necessary to stretch it to enhance its strength. However, the absolute difference .vertline.Tg.sub.1 -Tg.sub.2 .vertline. between the glass transition temperature of a clad material made of the copolymer of perfluoro[2,2-dimethyl-1,3-dioxole] (Tg.sub.2 (.degree.C )) and at least one other ethylenically unsaturated monomer results in that of this core material (Tg.sub.1 (.degree.C )) becomes too large, the stretching temperature of the optical fiber becomes too high for a clad material to be stretched, and the properties of this optical fiber being damaged.
In addition to this problem, a clad copolymer comprising perfluoro[2,2-dimethyl-1,3-dioxole] as the main monomer does not adhere well to a core material. This is a characteristic feature of a fluorine plastic. If an optical fiber having a core-clad two-layer structure containing this polymer as the core material is bent, the clad layer often separates from the core layer, and the plastic optical fiber decreases in flexibility. Such flexibility is a significant advantage of a plastic optical fiber.